A method for manufacturing a hollow part with a branch is known which includes (a) a resin filling step of filling a mold with a molten resin, the mold having a main cavity that has a hollowing piece placed therein, and a branch cavity that is formed so as to communicate with the main cavity, that has a slide shaft inserted therein so that the slide shaft can advance and withdraw therein, and that forms a branch pipe portion in an annular space between the branch cavity and the slide shaft, (b) a main pipe portion forming step of forming a hollow main pipe portion by moving the hollowing piece to force excess resin out of the main cavity through an outlet port formed at one end of the main cavity and thus form a hollow portion in the resin in the main cavity, and (c) a shaft removing step of withdrawing the slide shaft located in the branch cavity to form a hollow portion in the branch pipe portion, (d) the hollow part with a branch being made of a resin and having the branch pipe portion integrally connected to the main pipe portion such that the hollow portions communicate with each other. A technique described in Patent Document 1 is one example of such a method. Patent Document 1 proposes that a communication hole is formed in a joint between the main pipe portion and the branch pipe portion by drilling with a drill, a cutter, etc. or by turning a holding bar, which is formed integrally with a resin wall in a portion where the communication hole is to be formed, so as to remove the resin wall in this portion.